Our latest update on migration statistics welcomes a series of recent improvements in ONS’s measurement of this important set of numbers. But focusing on the short-term gains obscures a much bigger picture of progress. The Code of Practice for Statistics encourages the innovation and improvement of methods. The story of migration statistics over the last decade and a half illustrates this practice in action.
Let’s start with the most recent improvements. In a progress report we published in December 2023 on the Office for National Statistics (ONS)’s transformation of long-term international migration estimates, we set four requirements for change, asking ONS to:
- develop plans for the ongoing review of data sources
- develop and publish measures of uncertainty
- publish long-term plans for international migration statistics
- publish a road map for population and migration transformation
As our latest update shows, ONS has made progress on all of them.
This is clearly welcome and means that users can have more confidence in migration statistics.
There is of course more to do. We expect ONS to build on this progress, by continuing to:
- research and test methods to improve the estimates
- address remaining gaps, such as estimates for non-UK-born populations
- maintain user confidence in the relevance and quality of migration statistics
We are happy, however, to close our review formally: ONS has met the requirements we set and is clearly on the right track.
So, let’s step back. How far have migration statistics come in the last 15 years or so? Fifteen years ago, the measurement of migration depended almost entirely on a relatively small survey of passenger intentions, the International Passenger Survey (IPS).
In 2013, the Public Administration Select Committee (predecessor to the Public Administration and Constitutional Affairs Committee) published a report on migration statistics. Noting that the UK’s migration estimates were based on the IPS, which interviewed a small sample of people entering and leaving the UK, it found that the survey-based method for measuring migration was ‘not fit for purpose’:
“The International Passenger Survey is inadequate for measuring, managing and understanding the levels of migration that are now typical in the UK. The Government must plan to end reliance on the International Passenger Survey as the primary method of estimating migration: it is not fit for the purposes to which it is put.”
And in March 2016, on the eve of the EU referendum, I wrote to the then-National Statistician, John Pullinger, expressing concern about the divergence between ONS’s estimates of long-term international migration using the IPS and the evidence from the Department for Work and Pensions (DWP) National Insurance number registrations data:
“There is a significant risk that a lack of progress in reconciling and explaining the differences over the coming weeks could undermine public confidence in official migration estimates.”
ONS responded by publishing a Note on the difference between National Insurance number registrations and the estimate of long-term international migration, clarifying that the two sources measured different things: National Insurance number registrations included short-term as well as long-term migrants, while these ONS estimates focused solely on long-term migration where people changed their country of residence for 12 months or more.
Since then, there has been progress in migration statistics – at first slow and steady, and more recently dramatic and significant:
- The first major development was ONS increasing its use of Home Office border data to supplement the IPS.
- In 2019, we removed the accreditation from the migration figures, so that they were no longer accredited official statistics. This reflected the uncertainty surrounding estimates based on the IPS.
- The COVID-19 pandemic meant that it became much harder to conduct the IPS at the border.
- From 2020, the IPS ceased to be the main source of data for estimating migration.
Since the pandemic, ONS has placed ever greater reliance on administrative data, including Home Office visa records and DWP benefit records (as the basis for establishing whether someone has remained in the UK or not). The last component of the estimates that relied on the survey was the migration estimates for British nationals. This estimate switched over to an administrative basis in 2025.
As a result, the migration estimates now fully depend on administrative sources. There are several benefits to this, including much more comprehensive coverage (closer to 100% of people entering the country legally, as opposed to a far, far smaller percentage under the IPS); a greater ability to disaggregate the data into smaller groupings (for example, by different nationalities); and a much greater ability to validate assumptions about intention (by seeing, for example, whether people who receive visas take them up to enter the UK, and whether they bring dependents with them).
In short, the ambition set out by the Public Administration Select Committee in 2013, and confirmed by the Chair of the UK Statistics Authority that same year, has now been realised.
The changes have not come without challenge. In particular, ONS now has to make different types of revisions to its migration estimates and these shouldn’t all be interpreted in the same way. Some revisions happen because, when someone enters the country, ONS needs to make an assumption as to whether they will stay for 12 months (the United Nations definition of an international migrant) or not. ONS makes this assumption based on past patterns of migration, but the assumptions can be and are superseded by actual evidence as people either stay beyond 12 months or leave. Other revisions come from planned improvements to data sources or methods.
As a result, ONS has to make revisions to its preliminary estimates. For example, when ONS finalised provisional estimates in May 2026, net migration was revised downwards by 14,000 for the year ending December 2024 and upwards by 15,000 for the year ending June 2025 (see ONS’s long-term migration report, published in May 2026).
But stepping back from the latest changes in the estimates, ONS’s progress over the last decade is not just at the margin. It is a complete overhaul of data sources and methods, and a strengthened approach to quality assurance. I think the extent of this change has been missed by many.
The current figures are not perfect. They are not free from uncertainty. There is plenty more to do, especially about communicating uncertainty and revisions. In addition, measurement of the migration of British nationals remains the most challenging aspect of migration to measure; and while measures of the flow of migrants have improved, there is still a user need for better measures of the stock of migrants in the UK. But in an environment where the UK statistical system as a whole is sometimes criticised for not making full use of administrative data, the story of migration statistics provides clear evidence of progress.